how to resolve the error of getcontext

how to resolve the error of getcontext

I am using HTML5 Canvas,javascript.i have made an page with the help of
html5 canvas and javascript..But when i run it it gave me this error
("Error=Unable to get the value of the property 'getContext': object is
null or undefined")
but it does not terminate the program..the program is running fine with
this .I dont know what to do to remove this thing..here is that line in
which it is giving error
var canvas = canvas0;
options = null;
// Canvas good?
if (canvas !== null && canvas.getContext)
and if any changes i bring to canvas.getContext than it give error on
running the page that canvas is not supported..How to resolve this
problem..